New materials with high dielectric constants and low loss tangents are needed in the electronics industry for use at high frequencies and as a means to enable further miniaturization. These materials are particularly useful if they can be made into thin films, sheets, plaques, and other molded shapes, so that they can be used as circuit boards at microwave frequencies, high energy density capacitors, filters, antennas, buried components, and multichip modules. These have a variety of end uses, as for example in wireless communications. Many ceramic materials have the desired high dielectric constant and low dielectric loss, but they are not readily made into thin films. Ceramic materials that have been fabricated into films and shaped articles are also generally brittle.
One approach to making films and sheets with the desired properties is to utilize a composition (also referred to herein as "composite") comprising a polymeric matrix and a ceramic filler having a high dielectric constant. This approach is difficult because the compositions need high levels of the ceramic filler in order to achieve the desired high dielectric constant while retaining rheological properties that make the compositions suitable for extrusion or molding. The compositions must also be stable to changes in ambient moisture (humidity) and temperature. Resistance to elevated temperatures, as well as high mechanical strength, impact resistance and chemical resistance are also all desirable. Finally, in many applications, flat substrates made from these materials will need to be made into laminates with copper and/or other materials.
Several high dielectric constant materials based on polymers combined with ceramics are known. For example, numerous patents are assigned to Rogers Corporation that teach composites of fluoropolymers, preferably poly(tetrafluoroethylene) (PTFE), and ceramic materials for use as high dielectric materials, as for example U.S. Pat. Nos. 4,335,180 and 5,358,775. Rogers Corporation sells some of these compositions of PTFE and ceramic fillers for use as high dielectric films. It is in general difficult to make thin films and other shaped articles of PTFE containing a filler.
A large number of other examples of high dielectric composite materials have been disclosed. Typical examples are U.S. Pat. No. 5,174,973, German patent publication DE 3,242,657, Japanese Patent Publications JP 5,307,911, JP 57,853, JP 98,069 and a published paper (S. Asai, et al., IEEE Transactions on Components, Hybrids and Manufacturing Technology, Vol. 16, No. 5, August, 1993, pp. 499-504).
The temperature stability of the dielectric constant is an important issue. The dielectric constants of high dielectric ceramics change with temperature. As a result, the dielectric constants of compositions of polymers and ceramics also change with temperature. The changes in dielectric constant affect the electrical properties of electronic components utilizing the compositions, such as, for example, the resonant frequency of a patch antenna. This may limit the usefulness of the devices, since they are only usable within limited temperature ranges. Outdoor use may be particularly unreliable. This problem has so far been addressed by making compositions that contain combinations of ceramic materials that have positive and negative temperature coefficients of dielectric constant, so that the changes in dielectric constant of the materials balance each other out. This approach is described in U.S. Pat. No. 5,552,210, Japanese Patent Publication No, 4,161,461 published in 1992, and commonly assigned co-pending U.S. application Ser. No. 08/646,403. An alternative approach is to make a composition in which the ceramic filler has a temperature coefficient of dielectric constant that is approximately zero. Such ceramic fillers are known but are very costly. A new approach that utilizes common high dielectric constant ceramic materials and common polymers is described below.